Re: [FFmpeg-devel] [PATCH] avformat/mov: correct 0 valued entries in stts

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2021-12-28 12:38 am, Michael Niedermayer wrote:
> On Mon, Dec 27, 2021 at 11:27:10AM +0530, Gyan Doshi wrote:
>> As per ISO 14496-12, sample duration of 0 is invalid except for
>> the last entry.
>>
>> In addition, also catch 0 value for sample count.
>> ---
>>   libavformat/mov.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>> index 2aed6e80ef..fb7406cdd6 100644
>> --- a/libavformat/mov.c
>> +++ b/libavformat/mov.c
>> @@ -2968,6 +2968,18 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>>           av_log(c->fc, AV_LOG_TRACE, "sample_count=%d, sample_duration=%d\n",
>>                   sample_count, sample_duration);
>>   
>> +        if (!sample_count) {
>> +        av_log(c->fc, AV_LOG_WARNING, "invalid sample count of 0 in stts for st %d at entry %u; changing to 1.\n",
>> +               c->fc->nb_streams-1, i);
>> +        sc->stts_data[i].count = sample_count = 1;
>> +        }
>> +
>> +        if (!sample_duration && i != entries-1) {
>> +        av_log(c->fc, AV_LOG_WARNING, "invalid sample delta of 0 in stts for st %d at entry %u; changing to 1.\n",
>> +               c->fc->nb_streams-1, i);
>> +        sc->stts_data[i].duration = sample_duration = 1;
>> +        }
>> +
>>           duration+=(int64_t)sample_duration*(uint64_t)sample_count;
>>           total_sample_count+=sample_count;
> This does not produce the same output
> tickets/2096/m.f4v
>
> videos/stretch.mov (2344 matches for "invalid" after this patch)
>
> tickets/976/CodecCopyFailing.mp4
>
> But there are many more, some maybe even generated by FFmpeg

Where do I find these files?

> Taking a step back, the problem started with
> 203b0e3561dea1ec459be226d805abe73e7535e5
> which broke a real world file which was outside the specification

Just to clarify, it did not break that file. That file uses stts in an 
unusual way.
Before 2015, lavf exported packets with the same desync as the other 
demuxers do so till today.
Andreas' patch added a hack to make it play in sync. My patch 203b0e356 
broke that hack.
The patch for max_stts_delta is a way to restore it back.

> you then suggested a fix which crashed with some fuzzed files which
> where outside the specification
>
> and now this fix on top which changes real world files which
> are outside the specification
>
> I think, maybe you should consider the "outside the specification"
> more. The code above directly and intentionally changes values.
> So as a reviewer i have to ask the obvious, is that change a
> bugfix or a bug ?

Not surprising that the output of out-of-spec files is different - 
that's expected, intended and trivial.
It would be a bug if in-spec files were treated differently. FATE passes.

If there's a specific / "correct" playback for these files like sync 
issues, I'll see if I can restore it.

Regards,
Gyan
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